1. Field of the Invention
The present invention is generally related to data transfer apparatuses, more particularly, to data transfer apparatuses for low voltage differential signaling.
2. Description of the Related Art
Low voltage differential signaling (LVDS) is a well-known technique for achieving high-speed data transmission with reduced electromagnetic interference (EMI). In order to suppress EMI, LVDS-based data transmission apparatuses use differential signals having reduced amplitudes as far as data are successfully transferred.
Pre-emphasis is known in the art as one of the techniques for improving reliability of LVDS data transmission. Japanese open Laid Patent Application No. P2002-368600A discloses a pre-emphasis circuit for boosting the amplitudes of edge portions of differential signals.
Adjusting amplitudes of differential signals is another technique for improving data transmission reliability. Data transmission based on LVDS requires appropriate adjustment of amplitudes of differential signals, especially for the case when lengths of transmission lines between sending and receiving entities, and for the case when the transmission lines experience considerable characteristics variances.
Japanese Open Laid Patent Application No. P2000-341177A discloses an LVDS-based image data transmission apparatus that manually controls the amplitudes of differential signals.
Japanese Open Laid Patent Application No. P2001-339315 discloses another LVDS-based image data transmission apparatus that dynamically controls the amplitudes of differential signals.
As illustrated in FIG. 1, the conventional image signal transmission apparatus is used for transferring image data from a personal computer 100 to a liquid crystal projector 200. The personal computer 100 includes a graphics controller 101, a sending unit 102, a main CPU 106, and a bus 107, while the liquid crystal projector 200 includes a receiving unit 103, and a liquid crystal display panel 104. The sending unit 102 includes an encoder and parallel-serial converter 111, a PLL circuit 112, and an amplitude controller 113. Additionally, the sending unit 102 is connected with a variable resistance circuit 114. The receiving unit 103 includes serial-parallel converter and decoder 131, and a PLL circuit 132.
The sending unit 102 is designed to provide differential signals, including image data and control signals, and clock signals for the receiving unit 103 through transmission lines 104, each including a pair of signal lines.
In order to control the amplitude of the differential signals, the receiving unit 103 includes a coupler 141 connected to specific one of the transmission lines 104. As shown in FIG. 2, the coupler 141 is connected to one of the signal lines RXR+ and RXR− of the specific transmission line 104; it should be noted that the coupler 141 is not connected to both of the signal lines RXR+ and RXR−. A detected signal generated by the coupler 141 is demodulated by a demodulator 142, and then analog-digital converted by an A/D converter 143. The demodulated and analog-digital converted signal is fed back to the personal computer 100 through a CPU 144, and a line driver 145. The sending unit 102 within the personal computer 100 is responsive to the feedback signal received from the liquid crystal projector 200 for controlling the amplitudes of the differential signals transmitted to the receiving unit 103.
One drawback of the conventional technique is that the architecture using the coupler 141 and the demodulator 142 undesirably experiences increased signal loss. Disposing the coupler 141 inevitably affects the impedance matching of the transmission line 104 connected to the coupler 141, and thus causes an increase in the signal loss. The increased signal loss undesirably deteriorates the detection accuracy of the amplitudes. This problem is especially serious if the transmission lines 104 experiences increased loss.
Another drawback is that the aforementioned architecture enhances complexity and size of the circuitry used for the detection of the amplitudes, because the architecture requires a high-speed amplifier adapted to high frequency ranges of the LVDS signals. Typical demodulators include diodes, which have dead band of several hundred millivolts. Therefore, detecting the amplitude of the differential signal using the demodulator 142 requires an amplifier for amplifying the detected signal from the coupler 141; however, the amplifier is required to adapt extremely high frequencies because the differential signal typically ranges between 100 MHz and 10 GHz. This undesirably enhances complexity and size of the required amplifier.
Still another drawback of the architecture shown in FIG. 1 is that the architecture does not deal with common mode noise, because the coupler 141 is coupled to only one of the pair of the signal lines.
These drawbacks prevent desirable feedback control of the amplitude of the differential signal.
Therefore, there is a need for providing a data transmission apparatus for achieving improved feedback control of an amplitude of an LVDS signal with simplified circuitry configuration.